[Python] Catch PermissionDenied when checking the feature store in VertexAIFeatureStoreEnrichmentHandler

[schizophrenicmaniac]

VertexAIFeatureStoreEnrichmentHandler.__init__ tries to check that the feature store exists and is meant to only log a warning if the caller doesn't have admin permission. But the try/except only covered creating FeatureOnlineStoreAdminServiceClient, which makes no RPC. The call that actually checks permissions, get_feature_online_store, was in the else branch, so a PermissionDenied went uncaught and failed pipeline construction for users who only have data read access.

This moves the path building and get_feature_online_store into the try and catches PermissionDenied specifically, so NotFound and other errors still surface. Errors raised while creating the client are no longer turned into the "insufficient admin permission" warning either. They would fail again in __enter__ anyway. Added unit tests that mock the admin client for the PermissionDenied and NotFound cases.

Fixes #40442

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.01%. Comparing base (69bdf5f) to head (e2a7473).
⚠️ Report is 44 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40463      +/-   ##
============================================
- Coverage     59.04%   59.01%   -0.03%     
+ Complexity    15624    15616       -8     
============================================
  Files          2797     2798       +1     
  Lines        280750   280843      +93     
  Branches      12488    12477      -11     
============================================
- Hits         165764   165747      -17     
- Misses       108540   108655     +115     
+ Partials       6446     6441       -5     

Flag	Coverage Δ
python	79.55% <100.00%> (-0.11%)	⬇️

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